You may freely paraphrase, link to or quote portions of any unlocked entry, or any locked entry in a community with open membership. If the entry is locked and in a closed community, you need permission from the original poster to link or quote (paraphrasing is okay). You also need permission to link or quote any individual user's LJ (paraphrasing is all right). Under no circumstances may you identify the individual user without permission, even if you're linking to their post in an open community -- target the post's contents, not the poster.
This rule will appear in a shorter form on the community's info page.